Stjepana Kovac

Nrf2 regulates ROS production by mitochondria and NADPH oxidase.

Background Nuclear factor (erythroid-derived 2) factor 2 (Nrf2) is a crucial transcription factor mediating protection against oxidants. Nrf2 is negatively regulated by cytoplasmic Kelch-like ECH associated protein 1 (Keap1) thereby providing inducible antioxidant defence. Antioxidant properties of Nrf2 are thought to be mainly exerted by stimulating transcription of antioxidant proteins, whereas its effects on ROS production within the cell are uncertain. Methods Live cell imaging and qPCR in brain hippocampal glio-neuronal cultures and explants slice cultures with graded expression of Nrf2, i.e. Nrf2-knockout (Nrf2-KO), wild-type (WT), and Keap1-knockdown (Keap1-KD). Results We here show that ROS production in Nrf2-KO cells and tissues is increased compared to their WT counterparts. Mitochondrial ROS production is regulated by the Keap1–Nrf2 pathway by controlling mitochondrial bioenergetics. Surprisingly, Keap1-KD cells and tissues also showed higher rates of ROS production when compared to WT, although with a smaller magnitude. Analysis of the mRNA expression levels of the two NOX isoforms implicated in brain pathology showed, that NOX2 is dramatically upregulated under conditions of Nrf2 deficiency, whereas NOX4 is upregulated when Nrf2 is constitutively activated (Keap1-KD) to a degree which paralleled the increases in ROS production. Conclusions These observations suggest that the Keap1–Nrf2 pathway regulates both mitochondrial and cytosolic ROS production through NADPH oxidase. General significance Findings supports a key role of the Keap1–Nrf2 pathway in redox homeostasis within the cell.

Prolonged seizure activity impairs mitochondrial bioenergetics and induces cell death.

The mechanisms underlying neuronal death following excessive activity such as occurs during prolonged seizures are unclear, but mitochondrial dysfunction has been hypothesised to play a role. Here, we tested this with fluorescence imaging techniques in rat glio-neuronal neocortical co-cultures using low Mg2+ levels to induce seizure-like activity. Glutamate activation of NMDA receptors resulted in Ca2+ oscillations in neurons and a sustained depolarisation of the mitochondrial membrane potential, which was cyclosporine A sensitive, indicating mitochondrial permeability and transition pore opening. It was also dependent on glutamate release and NMDA receptor activation, because depolarisation was not observed after depleting vesicular glutamate with vacuolar-type H+-ATPase concanamycin A or blocking NMDA receptors with APV. Neuronal ATP levels in soma and dendrites decreased significantly during prolonged seizures and correlated with the frequency of the oscillatory Ca2+ signal, indicative of activity-dependent ATP consumption. Blocking mitochondrial complex I, complex V or uncoupling mitochondrial oxidative phosphorylation under low-Mg2+ conditions accelerated activity-dependent neuronal ATP consumption. Neuronal death increased after two and 24 hours of low Mg2+ levels compared with control treatment, and was reduced by supplementation with the mitochondrial complex I substrate pyruvate. These findings demonstrate a crucial role for mitochondrial dysfunction in seizure-activity-induced neuronal death, and that strategies aimed at redressing this are neuroprotective.

Patterns of neurotransmitter receptor distributions following cortical spreading depression

Spreading depression (SD), a self-propagating depolarization of neurons and glia, is believed to play a role in different neurological disorders including migraine aura and acute brain ischaemia. Initiation and propagation of SD modulate excitability of neuronal network. A brief period of excitation heralds SD which is immediately followed first by prolonged nerve cell depression and later by an excitatory phase. The aim of the present study was to characterize local and remote transmitter receptor changes after propagation of cortical SD. Quantitative receptor autoradiography was used to asses 16 transmitter receptor types in combined striatum-hippocampus-cortex slices of the rat 1 h after induction of cortical SD. In neocortical tissues, local increases of glutamate NMDA, AMPA, and kainate receptor binding sites were observed. In addition to up-regulation of ionotropic glutamate receptors, receptor binding sites of GABA(A), muscarinic M1 and M2, adrenergic alpha(1) and alpha(2), and serotonergic 5-HT(2) receptors were increased in the hippocampus. Cortical SD also upregulated NMDA, AMPA, kainate, GABA(A), serotonergic 5-HT(2), adrenergic alpha(2) and dopaminergic D1 receptor binding sites in the striatum. These findings indicate selective changes in several receptors binding sites both in cortical and subcortical regions by SD which may explain delayed excitatory phase after SD. Mapping of receptor changes by cortical SD increases our understanding of the mechanism of SD action in associated neurological disorders.

Neuropeptides in epilepsy

Neuropeptides play an important role in modulating seizures and epilepsy. Unlike neurotransmitters which operate on a millisecond time-scale, neuropeptides have longer half lives; this leads to modulation of neuronal and network activity over prolonged periods, so contributing to setting the seizure threshold. Most neuropeptides are stored in large dense vesicles and co-localize with inhibitory interneurons. They are released upon high frequency stimulation making them attractive targets for modulation of seizures, during which high frequency discharges occur. Numerous neuropeptides have been implicated in epilepsy; one, ACTH, is already used in clinical practice to suppress seizures. Here, we concentrate on neuropeptides that have a direct effect on seizures, and for which therapeutic interventions are being developed. We have thus reviewed the abundant reports that support a role for neuropeptide Y (NPY), galanin, ghrelin, somatostatin and dynorphin in suppressing seizures and epileptogenesis, and for tachykinins having pro-epileptic effects. Most in vitro and in vivo studies are performed in hippocampal tissue in which receptor expression is usually high, making translation to other brain areas less clear. We highlight recent therapeutic strategies to treat epilepsy with neuropeptides, which are based on viral vector technology, and outline how such interventions need to be refined in order to address human disease.

Dopamine protects neurons against glutamate-induced excitotoxicity

Glutamate excitotoxicity is responsible for neuronal death in acute neurological disorders including stroke, trauma and neurodegenerative disease. Loss of calcium homeostasis is a key mediator of glutamate-induced cell death. The neurotransmitter dopamine (DA) is known to modulate calcium signalling, and here we show that it can do so in response to physiological concentrations of glutamate. Furthermore, DA is able to protect neurons from glutamate-induced cell death at pathological concentrations of glutamate. We demonstrate that DA has a novel role in preventing delayed calcium deregulation in cortical, hippocampal and midbrain neurons. The effect of DA in abolishing glutamate excitotoxicity can be induced by DA receptor agonists, and is abolished by DA receptor antagonists. Our data indicate that the modulation of glutamate excitotoxicity by DA is receptor-mediated. We postulate that DA has a major physiological function as a safety catch to restrict the glutamate-induced calcium signal, and thereby prevent glutamate-induced cell death in the brain.

Seizure activity results in calcium- and mitochondria-independent ROS production via NADPH and xanthine oxidase activation

Seizure activity has been proposed to result in the generation of reactive oxygen species (ROS), which then contribute to seizure-induced neuronal damage and eventually cell death. Although the mechanisms of seizure-induced ROS generation are unclear, mitochondria and cellular calcium overload have been proposed to have a crucial role. We aim to determine the sources of seizure-induced ROS and their contribution to seizure-induced cell death. Using live cell imaging techniques in glioneuronal cultures, we show that prolonged seizure-like activity increases ROS production in an NMDA receptor-dependent manner. Unexpectedly, however, mitochondria did not contribute to ROS production during seizure-like activity. ROS were generated primarily by NADPH oxidase and later by xanthine oxidase (XO) activity in a calcium-independent manner. This calcium-independent neuronal ROS production was accompanied by an increase in intracellular [Na+] through NMDA receptor activation. Inhibition of NADPH or XO markedly reduced seizure-like activity-induced neuronal apoptosis. These findings demonstrate a critical role for ROS in seizure-induced neuronal cell death and identify novel therapeutic targets.

The LaLiMo Trial: lamotrigine compared with levetiracetam in the initial 26 weeks of monotherapy for focal and generalised epilepsy—an open-label, prospective, randomised controlled multicenter study

Background Of the newer antiepileptic drugs, lamotrigine (LTG) and levetiracetam (LEV) are popular first choice drugs for epilepsy. The authors compared these drugs with regard to their efficacy and tolerability in the initial monotherapy for epilepsy. Methods A randomised, open-label, controlled, parallel group, multicenter trial was conducted to test the superiority of the LEV arm over the LTG arm. The primary endpoint was the rate of seizure-free patients in the first 6 weeks (two-sided Fishers exact test, α=0.05, intent-to-treat set). Furthermore, efficacy, tolerability and quality of life were evaluated. The authors included 409 patients aged ≥12 years with newly diagnosed focal or generalised epilepsy defined by either two or more unprovoked seizures or one first seizure with high risk for recurrence. Patients were titrated to 2000 mg/day of LEV or 200 mg/day of LTG reached on day 22 or 71, respectively. Two dose adjustments by 500/50 mg were allowed. Results The proportions of seizure-free patients were 67.5% (LEV) versus 64.0% (LTG) 6 weeks after randomisation (p=0.47), and 45.2% (LEV) versus 47.8% (LTG) during the whole treatment period of 26 weeks. The HR (LEV vs LTG) for seizure-free time was 0.86 (95% CI, 0.61 to 1.22). Adverse events occurred in 74.5% (LEV) versus 70.6% (LTG) of the patients (p=0.38). Adverse events associated with study discontinuation occurred in 17/204 (LEV) versus 8/201 (LTG) patients (p=0.07). Conclusions There were no significant differences with regard to efficacy and tolerability of LEV and LTG in newly diagnosed focal and generalised epilepsy despite more rapid titration in the LEV arm. Clinical trial registration number ClinicalTrials.gov identifier NCT00242606.

Gelastic seizures: A case of lateral frontal lobe epilepsy and review of the literature

We describe a 40-year-old patient with gelastic seizures triggered by hand movement. Despite nonlesional magnetic resonance imaging (MRI), electroencephalography (EEG), functional magnetic resonance imaging (fMRI), and diffusion tensor imaging (DTI) are concordant with seizure onset in the right frontocentral area. Seizure semiology and EEG recordings imply involvement of mesial frontal structures remote from seizure initiation site. We reviewed all published cases on gelastic seizures of frontal lobe origin to find characteristic features. For further investigation of the phenomenon of movement-induced seizures, fMRI was performed using a finger tapping paradigm. Interictal fMRI revealed widespread activation of right motor cortex during finger tapping on either side outreaching the anatomical representation of the left finger. In line with this finding DTI revealed fiber track impairment in the right frontocentral region, supporting the hypothesis of a focal derangement. This case highlights the importance of complementary functional investigations in MRI-negative epilepsies.

Spreading depression triggers ictaform activity in partially disinhibited neuronal tissues

There is unequivocal electrophysiological evidence that spreading depression (SD) can trigger epileptiform field potentials. In vitro experiments on human brain tissues indicated that γ-aminobutyric acid (GABA)-mediated inhibition prevented this process. Intra- and extracellular recordings of bioelectrical activities were performed in the rodent neocortex, hippocampus and amygdala after perfusion of low concentrations of the GABAA antagonist bicuculline and induction of SD by KCl application. Induction of SD in combined amygdala-hippocampus-cortex slices pre-treated with low concentration of bicuculline triggered epileptiform burst discharges in cortical as well as subcortical brain structures. Propagation of SD significantly depolarized the membrane, decreased the amplitude and duration of action potentials (APs) and after-hyperpolarization as well as the neuronal membrane input resistance and the amplitude of threshold potentials. Ten to twenty minutes after induction of SD, the pattern of APs changed from regular firing to a series of APs riding on an underlying paroxysmal depolarization shift before the appearance of typical ictaform activities. Changes of characteristic features of APs occurred after SD persisted during the appearance of epileptiform activities. These results indicate that SD increases neuronal excitability and facilitates synchronization of neuronal discharges in the presence of partial disinhibition of neuronal tissues. Our findings might explain the occurrence of seizures in neurological disorders with partial impairment of inhibitory tone, such as brain ischemia and epilepsy.

Lipid peroxidation is essential for phospholipase C activity and the inositol-trisphosphate-related Ca2+ signal

ABSTRACT Reactive oxygen species (ROS) are produced in enzymatic and non-enzymatic reactions and have important roles in cell signalling but also detrimental effects. ROS-induced damage has been implicated in a number of neurological diseases; however, antioxidant therapies targeting brain diseases have been unsuccessful. Such failure might be related to inhibition of ROS-induced signalling in the brain. Using direct kinetic measures of lipid peroxidation in astrocytes and measurements of lipid peroxidation products in brain tissue, we here show that phospholipase C (PLC) preferentially cleaves oxidised lipids. Because of this, an increase in the rate of lipid peroxidation leads to increased Ca2+ release from endoplasmic reticulum (ER) stores in response to physiological activation of purinoreceptors with ATP. Both vitamin E and its water-soluble analogue Trolox, potent ROS scavengers, were able to suppress PLC activity, therefore dampening intracellular Ca2+ signalling. This implies that antioxidants can compromise intracellular Ca2+ signalling through inhibition of PLC, and that PLC plays a dual role – signalling and antioxidant defence.